Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed enclosure or housing to an external point outside the enclosure. Implantable medical devices (IMDs) such as implantable pulse generators (IPGs) for cardiac pacemakers, implantable cardioverter defibrillators (ICDs), nerve, brain, organ and muscle stimulators and implantable gastric monitors, or the like, employ such electrical feedthroughs through a hermetically-sealed enclosure that surrounds operative internal circuitry and electrically couples said circuitry with external medical electrical leads and associated electrodes.
Such feedthroughs typically include a ferrule adapted to fit within an opening in the enclosure, one or more conductor and a non-conductive hermetic glass or ceramic seal which supports and electrically isolates each such conductor from the other conductors passing through it and from the ferrule. The IMD enclosure is formed of a biocompatible metal such as titanium, although non-conductive ceramic materials have been proposed for forming the enclosure. The ferrule is typically of a metal that can be welded or otherwise mechanically coupled to the enclosure in a hermetically sealed manner.
Initially, single pin feedthroughs supported by glass, sapphire and ceramic were used with hermetically sealed IPGs. As time has passed, the volume and, consequently the surface area, of enclosures for IMDs has dramatically decreased and the number of medical electrical leads and associated electrodes coupled with the internal circuitry of an IMD has increased. Consequently, use of the relatively large single pin feedthroughs was no longer feasible, and numerous multiple conductor feedthroughs have been used or proposed for use that fit within the smaller sized opening and provide two, three, four or more conductors. Examples include those depicted and described in U.S. Pat. Nos. 6,660,116; 6,414,835; and 5,870,272 the contents of which are hereby incorporated herein by reference.
Many different insulator structures and conductor structures are known in the art of multiple conductor feedthroughs wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids through the feedthrough and into the housing of the medical device. The conductors typically comprise electrical wires or pins that extend through a glass and/or ceramic layer within a metal ferrule opening as shown, for example, in commonly assigned U.S. Pat. Nos. 4,991,582; 5,782,891; and 5,866,851 or through a ceramic enclosure as shown in the commonly assigned '891 patent and in U.S. Pat. No. 5,470,345.
Such multi-conductor feedthroughs have an internally disposed portion configured to be disposed inside the enclosure for connection with electrical circuitry and an externally disposed portion configured to be disposed outside the enclosure that is typically coupled electrically with connector elements for making connection with the leads, electrodes or sensors. The elongated lead conductors extending from the connector elements effectively act as antennae that tend to collect stray electromagnetic interference (EMI) signals that may interfere with normal IMD operations. At certain frequencies, for example, EMI can be mistaken for programming signals from an external programming device that can then cause an IMD to change operating mode.
This problem has been addressed in certain of the above-referenced patents by incorporating a capacitor structure upon the internally facing portion of the feedthrough ferrule coupled between each feedthrough conductor and a common ground, the ferrule, to filter out any high frequency EMI transmitted from the external lead conductor through the feedthrough conductor. The feedthrough capacitors originally were discrete capacitors but presently can take the form of chip capacitors that are mounted as shown in the above-referenced '891, '435, '476, and '906 patents and in further U.S. Pat. Nos. 5,650,759; 5,896,267; and 5,959,829, for example. Or, the feedthrough capacitors can take the form of discrete discoidal capacitive filters or discoidal capacitive filter arrays as shown in commonly assigned U.S. Pat. Nos. 5,735,884; 5,759,197; 5,836,992; 5,867,361; and 5,870,272 and further U.S. Pat. Nos. 5,287,076; 5,333,095; 5,905,627 and 5,999,398. These patents disclose use of discoidal filters and filter arrays in association with conductive pins which are of relatively large scale and difficult to miniaturize without complicating manufacture. It is desirable to further miniaturize and simplify the fabrication of the multi-conductor feedthrough assembly.
A high integrity hermetic seal for medical implant applications is very critical to prevent the ingress of body fluids into the IMD. Even a small leak rate of such body fluid penetration can, over a period of many years, build up and damage sensitive internal electronic components. This can cause catastrophic failure of an IMD. The ferrule of a feedthrough for an IMD typically is selected from a material having physical properties that match the properties of the surrounding IMD enclosure. For example, if an IMD enclosure is constructed from grade 2 titanium, a feedthrough ferrule of grade 2 titanium is implemented. However, as the dimensions of such prior art feedthroughs have expanded, combined with the high temperature they are typically subjected to during fabrication the inventors found that the grade 2 titanium oftentimes fails to maintain the close dimensional tolerance required to guarantee long-term hermeticity. That is, the grade 2 titanium metal component of the ferrule changes shape which potentially undermines the close fit of the ferrule in the corresponding aperture of the enclosure thereby negatively effecting the ability to repeatably join the feedthrough to the enclosure via welding. Furthermore, the inventors posit that when subjected to high temperature brazing to seal the interface between a ferrule and an insulator member such relatively high-grade titanium undergoes random dimensional distortion. This random distortion can depend upon such factors as the manner of fabrication of the high grade titanium (e.g., how it was initially forged, drawn into sheet form, etc.) and thus is essentially unpredictable. Furthermore, the inventors suggest that at least part of the reason that grade 2 titanium distorts relates to the physical composition thereof. That is, grade 2 titanium comprises relatively small sized grains and when heated up to or in excess of the phase transition temperature (approximately 850 degrees Celsius) the coupling between the grade 2 titanium grains changes and such changes can lead to a loss of the originally specified dimensions of grade 2 titanium components. The phase transition temperature increases with increasing impurity content, (e.g., grade 4 has a transition temperature of approximately 950 degrees Celsius).
Thus, the inventors have recognized a need in the art to fabricate highly accurately dimensioned components that are integral to high-grade titanium IMD enclosures so that long-term hermeticity is maintained. In particular, the inventors have discovered that as the overall surface area of diverse IMDs has decreased relative to the size of certain integral IMD-surface components (e.g., multi-polar feedthrough assemblies), maintaining dimensional is critical to maintaining long-term hermeticity and to the ability to mount accessories directly to the feedthrough, such as, but not limited to modules for electronic assembly or multi-hole EMI filter capacitors contained in a single dielectric structure. In addition, the inventors has discovered that, in addition to the potential negative impact to long-term fit issue with the enclosure & with components added to the feedthrough, the traditional grade-matching (i.e., grade 2 titanium enclosure and grade 2 titanium ferrule) reduces manufacturing yield of acceptable feedthrough (and ferrule) components and finally assembled IMDs.
Moreover, highly accurately dimensioned ferrules lead to robust feedthrough arrays of simplified construction, utilizing straightforward and uncomplicated assembly, thus resulting in overall manufacturing cost reductions. The inventors contemplate applying the present invention to myriad multi-polar feedthrough assemblies, including those designed for effectively filtering out undesirable EMI. The present invention fulfills these needs and provides other related advantages which will be appreciated by those skilled in the art as defined by the appended claims.